1. Field of the Invention
The present invention relates to oversampled, noise-shaping signal processing circuits, and in particular, to noise-shaped, high-efficiency amplifiers which convert an analog input signal to a pulse-width-modulated output signal.
2. Description of the Related Art
Referring to FIG. 1, a conventional high-efficiency amplifier system 10 includes, in various forms, a modulator 12, an amplifier 14 and a filter 16, interconnected substantially as shown. The analog input signal 11 is modulated by the modulator 12 and the modulated signal 13 is amplified by the amplifier 14 which is typically a switching circuit so as to maintain high-efficiency. The amplified signal 15 is filtered by the filter 16 to produce the desired analog output signal 17. The amplifier 14 is a switching amplifier so as to minimize power dissipation, thereby allowing the modulator 12 and amplifier 14 to be integrated within a single integrated circuit 18.
One problem associated with such a system 10 is that of power supply ripple being coupled into the output signal 17 through the amplifier 14. Accordingly, techniques are needed to increase power supply rejection, particularly when using switching amplifiers.
Referring to FIG. 2, one technique involves noise shaping the output signal spectrum. This is done by feeding back the output signal 15 from the amplifier 14 via a feedback network 22. The resulting feedback signal 23 is then differentially summed with the input signal 11 in a signal summer 24. The resulting sum signal 25 is modulated by the modulator 12, rather than the original input signal 11.
Alternatively, the filtered output signal 17 from the filter 16 can be fed back instead of the non-filtered signal 15. However, the problem associated with feeding back the filtered output signal 17 is the difficulty in creating a system which is stable and still has a high gain and wide bandwidth. This problem arises due to the fact that the filter 16 is typically a second order filter with a natural, or cutoff, frequency which is just outside the bandwidth of interest. The two poles associated with this filter 16 restrict the open loop unity crossover bandwidth of the overall loop to the cutoff frequency, which is approximately the signal bandwidth. This means that the in-band gain cannot be very large. As a result, suppression of power supply ripple, switching noise and other undesirable in-band signals is generally minimal.
On the other hand, feeding back the non-filtered signal 15, i.e., directly from the switching node output of the amplifier 14, the amount of feedback (i.e., with respect to gain and bandwidth) is less restricted thereby making it possible to use more sophisticated feedback techniques, such as noise shaping. Noise shaping techniques, sometimes referred to as delta-sigma techniques, allow for the selective reduction of quantization noise present at the switching output of the high-efficiency amplifier stage 14.
Referring to FIG. 3, a conventional delta-sigma loop includes a serial configuration of integrators 32, 34, quantizer 36, amplifier 38, feedback network 40 and signal summing stages 42, 44 interconnected in a loop substantially as shown. (This delta-sigma loop configuration is illustrated as a second order configuration, but it will be readily understood that the techniques discussed herein can be scaled as desired to higher order loop configurations as well.) The feedback network 40 can implement various forms of feedback techniques.
For example, when the amplifier output 39 is to be fed back, and such output 39 is an analog signal, the feedback network 40 may include filtering when sampled integrators 32, 34 are used. Alternatively, if the integrators 32, 34 are continuous time integrators, the feedback network 40 need not necessarily include filtering, but may provide only continuous time gain as needed for the feedback signals 41a, 41b. Alternatively, if the quantizer output 37 is to be fed back, the feedback network 40 may include discrete time feedback, such as a digital-to-analog conversion function, so as to provide appropriate analog feedback signals 41a, 41b. Various combinations and permutations of these types of feedback for continuous time and sampled integrators are discussed in more detail in U.S. Pat. No. 5,777,512, the disclosure of which is incorporated herein by reference.
Problems associated with conventional noise-shaped, high-efficiency amplifiers, such as those discussed above, have involved the choice between providing a pulse density modulated (PDM) signal or a continuous pulse-width modulated (PWM) signal as the output. If a PDM signal is used, the resulting output signal has low distortion levels, but contains a high amount of in-band signal noise. Conversely, a continuous PWM signal has less in-band signal noise, but a higher degree of signal distortion. Accordingly, it would be desirable to provide a noise-shaped, high-efficiency amplifier system with less in-band signal noise than a PDM system and less signal distortion than a continuous PWM system.